This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. In studies performed during our previous proposals, a new methodology was developed for analyzing and interpreting iron L-edge transition intensity distributions in terms of the total and differential orbital covalency (DOC). It was found that the integrated iron L-edge intensity could be used to obtain the total covalency of the complex. Furthermore, a projection methodology was developed which allows the covalency of the individual symmetry-related sets of orbitals and the DOC to be experimentally determined from the L-edge multiplet intensity distribution. The previous study serves as a foundation for the series of studies proposed here. The methodology developed during the previous proposal will be expanded to complexes that exhibit metal-to-ligand charge transfer and complexes of non-cubic symmetry. The expanded methodology will be applied and calibrated to sigma-donor-pi-acceptor complexes, an essential step in the study of the nature of heme versus non-heme iron bonding and its relation to reactivity, a major focus of this proposal. Data will be obtained from a number of specific heme complexes and compared to the non-heme complexes of the previous study. Of special interest are the electronic structural differences amongst low-spin ferriheme systems of different ground state electronic configurations. Additionally, this proposed research will provide the groundwork for determining the electronic structure in a series of complexes that serve as models of the enzyme cytochrome c oxidase. The determination of covalency and DOC of these models should provide insight into the electronic structure of this enzyme and differences related to the O-O bond cleavage in the enzyme but not the model. Finally, the proposed research will begin our study of the electronic structure of high valent Fe-porphyrins. Using two non-heme iron(IV) complexes to define the electronic contributions of the Fe(IV) ion, we will next study the electronic structure of a recent Fe(IV)=O model compound.